The potential threat of biological and chemical agent warfare is an ever-increasing national security concern. Of the known biological and chemical warfare agents it has been suggested that those capable of being deployed as aerosols are of greatest concern due to their ease and speed of dissemination over wide areas in lethal concentrations. All six of the Category A bioterrorism agents listed by the Centers for Diseases Control and Prevention are capable of being transmitted as bio-aerosols, including Bacillus anthracis, more commonly known as “anthrax.” The detection of such biological and chemical weapons attacks, however, is inherently difficult due to the small sample sizes involved. For example, a lethal dose of Bacillus anthracis spores weighs only 4 ng. In addition, these small samples can be widely dispersed within the air and may be found mixed with many other aerosol particles present in concentrations thousands of times larger than the bio-aerosol particle of interest. These demanding sampling conditions and other detection issues such as the unreliability of real time particle source analysis and identification have been problematic for the rapid and specific screening of packages, letters, baggage, passengers, and shipping containers for biological and/or chemical agents.
Various methods, including standard microbiology, molecular, and mass spectroscopy based approaches have been and are currently employed to characterize aerosol particles, including the detection of bio-aerosol particles and chemical agent aerosol particles. While such methodologies are often capable of providing species level detection of bio-aerosol particles, they are, however, typically achieved at the expense of long analysis times ranging from hours to days, when sample collection, preparation, and actual analysis/identification are all considered. For example, traditional microbiological methods such as culturing are time-consuming, labor-intensive, and also detect only live cells. Molecular based methods, such as the Polymerase Chain Reaction (PCR), in-situ hybridization, and immunoassays, are extremely sensitive and specific at the species level which identify the presence of harmful bio-aerosol particles, but also require time-consuming sample collection, specialized reagents, and processing prior to analysis. And mass spectrometry is well suited to the detection of biological agents due to its high information content and its inherent sensitivity to extremely small samples. However, current mass spectrometry approaches also suffer from relatively long analysis times due to the required sample collection, culturing, preparation, and analysis. In all of these methods, offline operation precludes true real-time analysis and onsite identification of particle source, including threat agents, and may present too substantial a disruption of commerce to be used as a pragmatic alternative. In fact, many “online” and “real time” particle detection and analysis systems simply provide sorting of spectral data into similar groups (e.g. via fuzzy logic algorithms) for subsequent visual identification by an expert user. They also typically consume large amounts of expensive consumables and are also incapable of determining the concentration of the biologics and therefore cannot determine if an infectious dose was encountered.
In the alternative, spectroscopic techniques, such as laser induced fluorescence are used for the instantaneous optical analysis of bio-aerosol particles. Unfortunately, while such techniques are reagentless and operate autonomously at high rep rates, the resulting fluorescence spectra suffers from a lack of specificity for biologics, i.e. contains too little information to differentiate some environmental particles from the organisms of interest. Consequently they have unacceptably high false alarm probabilities (Pfa), such as from soot and dust, and are incapable of identifying harmless biological aerosol particles from harmful ones (i.e. species-level differences between single cells). The lack of specificity for biological aerosols is due to the limited mass range (less than 600 daltons) and inhomogeneity in the desorption/ ionization laser.
To address these challenges and concerns, aerosol mass spectrometry systems, such as aerosol time of flight mass spectrometers (ATOFMS) of the type shown in U.S. Pat. No. 5,998,215 to Prather et al, have been developed to perform rapid single particle analysis by instantaneous mass spectrometric characterization of aerosol particles without using reagents or requiring sample preparation. While such systems provide relatively rapid analysis of particles in flight, they are however limited to applications in environments with, for example, less than 102 particles per liter of air of background particles. This is due to inherent inefficiencies in the system limiting the analysis rate to approximately two particles per second (e.g. activation cycling of ablation laser for mass spectrometry). Consequently, this speed limitation makes the use of conventional ATOFMS systems difficult for rapid, real-time detection and specific identity determination of biological aerosol particles, since small samples of bio-aerosol particles are often widely dispersed and mixed within mediums, such as air, containing large concentrations of background particles (e.g. 106 particles per liter of air). This is especially true in polluted environments such as urban and industrial settings as well as battlefield conditions. Such systems would thus be applicable for the rapid, real-time detection of bio-aerosol particles and chemical agent aerosols in relatively pristine environments only.
There is therefore a need for a real time particle detection system providing rapid or virtually instantaneous identification of a single aerosol particle from among known particle types or sources, and which goes beyond a simple determination of a particle's chemical composition from mass spectra. In addition, there is also a need for a system which addresses the need for both rapid and specific determination of biological and chemical warfare agents in sampling mediums, such as air, containing large concentrations of background particles. To this end, the ability to rapidly detect, screen, and target for mass spectrometric analysis only selected/qualifying biological and chemical aerosol particles within a complex mixture of background particles would aid in the detection and interdiction of bioterrorist attack in real and often heavily polluted environments.